Electron-emitting cathodes are employed on electric propulsion (EP) thrusters (1) to compensate for the emission of positive ions so that the vehicle remains electrically neutral, and (2) to sustain the discharge in plasma thrusters such as Hall and gridded ion engines. Traditionally, the technology used for electron emission has been the hollow cathode. Hollow cathodes are gas-fed devices, utilizing a small amount of propellant and onboard power to produce electron emission currents from a few Amps to a few tens of Amps. Reliable operation has been demonstrated for ˜10,000 hours.
Typical hollow cathodes, as used in 1-kW-class Hall and ion thrusters, consume approximately 5-10% of the total thruster propellant and electrical power. Because the cathode itself generates no thrust, the consumption of propellant and power causes a direct 5-10% reduction in propulsion system efficiency and specific impulse. Although the ˜10% performance impact of hollow cathodes is not negligible, it is tolerated for 1-kW-class devices because of the reliability of the technology. However, because hollow cathodes do not scale well to lower power, the associated efficiency losses become unacceptable as thruster size is reduced.
EP thrusters capable of operating efficiently at power levels less than 100 W can lead to the realization of fully functional micro- and nanosatellites. Research efforts toward this end include low-power ion thrusters, Hall thrusters, and Field-Emission Electric Propulsion (FEEP) systems. While some success has been achieved in scaling thruster technology to low power levels, the hollow cathode has shown itself not amenable to scaling. Thus, while a hollow cathode consuming ˜50 W of electrical power and 0.5 mg/s of propellant is only a ˜10% efficiency reduction for a 1-kW thruster system, the same cathode technology can easily represent an intolerable 50-100% efficiency reduction for EP systems using total power less than 100 W. Therefore, low-power EP systems would benefit from cathode technology that can produce sufficient electron emission while consuming little or no gas or electrical power.
In an effort to develop low-power EP systems compatible with micro- and nanosatellites, much research has focused in recent years on developing zero-flow, low-power “cold” cathodes based on the phenomenon of electron field emission. In field emission, electrons are extracted directly from a bulk solid material by an intense applied electric field at the solid-vacuum interface. The strength of the electric field must be sufficient to enable electron tunneling through the boundary potential via a process known as Fowler-Nordheim emission. Electric field strengths required for emission exceed 4×109 V/m.
The most promising field-emission technology appears to be the Spindt-type cathode. Spindt emitters rely on geometric enhancement of electric fields near sharp tips, where the field strength is inversely proportional to the tip radius. Microfabrication techniques have been used to demonstrate Mo and Si emitters with tip radii as small as 4 nm.
While Spindt-type field emitters have found widespread success in non-EP disciplines (e.g., flat panel video displays, microwave devices and electron microscopy systems), their application to the environments typical of EP thrusters has been somewhat less successful. In particular, it has proven very difficult to maintain the integrity of the fragile, nanometer-sized emitter tips in anything but ultra-high vacuum environments. When operated below 10−9 Torr, Spindt-type field emitters have demonstrated reliable operation and long life. However, when operated at elevated pressures (10−5 Torr), the tip becomes blunt and/or contaminated and the ability to emit acceptable electron beam current is compromised. There are three main causes of tip degradation: (1) chemical contamination from oxygen or other reactive gases; (2) sputter erosion from ion impacts; and (3) destruction of the tip due to catastrophic arcing to nearby surfaces and/or electrodes.
Various approaches have been used in an attempt to circumvent the tip degradation mechanisms. Because most EP systems use inert gases as propellant, the potential for chemical contamination occurs mainly during ground testing. While this is still a significant obstacle, careful testing protocols can avoid tip contamination. Sputter erosion, however, is a more serious problem. The emitted electron current will readily ionize any residual gas in the vicinity of the tip. The resulting ions will be accelerated back towards the emitter causing unavoidable sputter erosion of the tip. This effect is exacerbated in the environment of an EP thruster, where significant quantities of ambient plasma ions produced within and around the thruster will amplify tip erosion. Carefully designed multi-layer, multi-electrode extractor/gate/accelerator structures have been developed to shield emitter arrays from sputtering. Such electrode geometries have demonstrated a significant improvement in emitter lifetime, however sputter erosion arising from ions produced within the multi-electrode structure remains an issue. Attempts to reduce applied electrode voltages below the tip sputter threshold are accompanied by reduced emission. The issue of catastrophic arcing has been addressed by fabrication techniques that incorporate current-limiting features in the substrate. While such current-limiting architectures have proven effective for a range of operating conditions, arc failures are unavoidable in significantly high-pressure environments.
None of the currently proposed methods are capable of eliminating cathode failure as the result of tip degradation. The most accepted approach to reducing the risk of cathode failure has been the proposition of massively parallel arrays of closely packed emitter tips. Emitter lifetime is factored in to the number of tips required, and destroyed or degraded tips are replaced by available spares. Of course, this approach has geometric and practical constraints. Therefore, low-power EP systems would benefit from cathode technology that overcomes the problems associate with tip degradation.